A Utrecht researcher has shown that humans are not the only animals that make decisions based on evidence. It turns out that chimpanzees have the ability to assess the quality of evidence when making decisions.

Humans are often described as “rational animals” — creatures who make decisions by weighing evidence, at least in theory, as we observe the world around us today. Nonetheless, a key characteristic of rationality is selective revision of beliefs. This means that a rational creature doesn’t blindly change its beliefs, but only does so after weighing the strength or weakness of new evidence.

For a long time, this ability was thought to be exclusive to humans, until now. Hanna Schleihauf, a psychologist at Utrecht University, writes in the renowned journal Science that she has discovered this same ability in chimpanzees.

Although chimpanzees and other great apes have shown impressive performance in problem-solving and reasoning, it has long been assumed that they lack the metacognitive sophistication required for rational revision of beliefs. However, in a series of experiments, Schleihauf and her colleagues demonstrate that chimpanzees can rationally revise their beliefs.

Schleihauf worked at a chimpanzee reserve in Uganda, where she presented chimpanzees with two boxes at a time. One of the boxes contained hidden food. “We then first gave the chimpanzees either strong or weak evidence of the presence of food in the box. Strong evidence consisted, for example, of showing them what was in the box; weak evidence consisted of shaking the box so that the chimpanzee could only hear that something was inside.”

The chimpanzee indicated where it thought the food was hidden. The researcher then provided new evidence about the other box: evidence that was either stronger or weaker than the original evidence. Schleihauf said that “when the first clue was stronger, the chimpanzees tended to stick with their first choice. But when the first piece of evidence turned out to be weaker, they changed their choice. This suggests that they weighed up different pieces of evidence and adjusted their beliefs based on the strength of the evidence.”

In a follow-up experiment, the researchers took one step further: they tested whether chimpanzees are aware of the reasons behind their own beliefs. The researchers wanted to know whether chimpanzees can change their minds when they discover that a piece of evidence on which they based their belief turned out to be invalid.

Schleihauf and her colleagues did this, for example, by showing the chimpanzees that the “food” in the box was not real food, but only a picture. In this way, she demonstrated that a piece of evidence could also be invalidated. “The chimpanzees understood that they had been misled and that the original evidence was no longer valid.”

With her findings, Schleihauf reveals that rational thinking is not unique to humans and that some elements of rational thinking may not have originated in our species.